1. Field of the Invention
The present invention is directed to surface mount conductive adhesive composition. In particular, the present invention relates to particular mixtures of finely divided silver particles, an acrylated urethane resin or an acrylic resin, an acrylate monomer, and free radical initiator useful as surface mount conductive adhesive compositions for bonding electrical components onto flexible circuitry such as membrane switches.
2. Description of Related Art Including Information Disclosed Under 37 CFR .sctn.'s 1.97 and 1.98
Existing commercially available surface mount conductive adhesives are typically made from mixtures of epoxy resins, epoxy curatives, solvent and silver. When cured, these materials tend to be very brittle or exhibit a high modulus of elasticity. For printed wiring board (PWB) applications, since the substrate is rigid, such high modulus adhesives produce acceptable bonding between the electrical component and the substrate. However, for flexible circuits such as membrane switches, high modulus surface mount conductive adhesives may not be suitable because of cracking and loss of adhesion during flexing.
It has also been observed that epoxy based adhesives show poor compatibility with polymer thick film substrates based on vinyl resins in that during the curing step, the epoxy system reacts with the resin resulting in severe discoloration and loss in adhesion.
G. P. Nguyen, J. R. Williams, and Fredrick W. Gibson, "Conductive Adhesives", Circuits Assembly January 1993, pages 36-38 and 41 discuss the general expected properties of surface mount conductive adhesives for several applications.